Ever-increasing numbers of autism diagnoses have parents worried about a skyrocketing epidemic, and this week’s news may only drive alarm higher. Perhaps it shouldn’t.

In 2010, 1 in 68 (or 14.7 per 1,000) 8-year-olds had an autism spectrum disorder, the Centers for Disease Control and Prevention now estimates. That number is a substantial increase from 2008, which had an estimate of 1 in 88 (or 11.3 per 1,000).

But the numbers might not reflect a spike in actual cases. Instead, the rise might b...